FIG. 1 shows a typical column antenna 101 for use in mobile communication systems. The column antenna 101 comprises a plurality of antenna elements 1031 to 103N that are connected by means of a fixed or passive tapering network 105. The tapering network 105 is fed from a transmitter 107. The transmitter 107 comprises a power amplifier 109 that receives a signal that is to be transmitted from a transmitter processing circuit 111, which in turn receives the signal to be transmitted from base band processing unit 113. The power amplifier is biased by a bias control circuit 117, which is supplied by a fixed DC voltage 119. The function of the tapering network 105 is to feed each antenna element 1031 to 103N with the same or different amplitudes and/or phases in order to shape the combined antenna beam that is emitted by the column antenna 101.
A disadvantage of using such a tapering function in a column antenna 101 is that energy losses will occur in the passive components and transmission lines that constitute the tapering network 105, thus degrading power efficiency.
A further disadvantage is that the tapering function of the passive tapering network 105 is fixed, which does not therefore allow individual amplitude settings per antenna element to be changed during runtime without decreasing power efficiency, particularly over wide power ranges.
Yet another disadvantage is that, when adjusting the phases for each antenna element in traditional antenna systems, analogue phase shifting circuits are required. Such analogue phase shifting circuits tend to be inaccurate, and it is therefore difficult to ascertain how accurate the output signal correlates to a desired value.